The present invention relates to apparatus for separating and conveying sheets from a stack, this apparatus comprising vacuum-operated suction elements for lifting individual sheets from the stack.
Apparatus for separating stacked sheets is frequently used in conjunction with a machine, which is to be fed with individual sheets from a stack of sheets, so as to provide feeding of such sheets to the machine. The machines concerned may be simple machines fed with sheets, e.g. copying machines which usually contain a magazine with stacked copying sheets and which are to be fed with copying sheets one at a time in accordance with some specified feeding scheme, or more sophisticated machines which by means of one or more working operations convert or process premanufactured sheet blanks to finished products and which for the effective utilization of the capacity of the machine require continuous feeding of individual sheet blanks in rhythm with the working speed of the machine.
When it is a matter of feeding stacked sheets of thin paper or similarly flexible material, for example, into a copying machine, the sheet-separating arrangement used normally is of the kind which typically comprises a friction element on a reciprocating sheet-conveying arm and which, by means of the friction element, is pressed into a slip-free engagement with the top side of the uppermost sheet to be drawn by this against the sheet directly underneath it from the stack. This arrangement certainly is rapid and simple, but it happens not infrequently that not only the uppermost, but also one or more of the sheets directly underneath it follow along when the uppermost sheet is being removed. The tendency to such unintentional double-feeding of sheets increases for obvious reasons when particularly tightly packed stacks of sheets with coarse or rough surfaces are involved.
On the other hand, when it is a matter of stacked sheets of cardboard, carton or similar rigid and self-supporting material, a conventional arrangement is used instead of the kind which typically comprises vacuum-operated suction elements in the form of funnels, cups or similarly dished elements on movable lifting arms and which, by means of the suction elements in vacuum-engagement with the uppermost sheet, lifts the latter off the sheet stack. This type of sheet-separating arrangement presents less tendency to double-feed the sheets but, on the other hand, is technically more complicated than the known arrangement of the friction type and requires, among other things, a relatively complicated valve arrangement for controlling the vacuum operation of the suction elements in correct rhythm with the working motions of the lifting arms. Moreover, it is is somewhat slower than the former arrangement and frequently also space-demanding, if sufficient swing room for the movements of the lifting arms is to be provided. Furthermore, it needs regular supervision and ajustment of the suction elements which in most cases are liable to wear.